This invention relates to surface acoustic waves and more particularly to a continuously variable, rapid, electrical control of the velocity of a propagating surface acoustic wave in a waveguide.
Heretofore surface acoustic wave phase shifters have been described in an article, "A Surface Wave Electronically Variable Phase Shifter" by J. A. Alusow, B. E. Burke, and E. Stern; in 1971 IEEE Ultrasonics Symposium, paper p-9. In the described device, the surface wave velocity depends on the spacing between a gold film and a lithium niobate delay line. Limitations of the device include mechanical stability, and slow response time since the variation is provided by a mechanical means. Further, the device requires close fabrication tolerances.
Another prior art system has been set forth in U.S. Pat. No. 3,827,002. In the patented device a difficulty arises in producing piezoelectric semiconductors with acceptable, uniform, semiconducting properties in the range required for phase shift applications; and a leaky current through the Schottky barrier heats the device creating unwanted thermal effects.